As wireless communications technologies increasingly develop, wireless communications base stations are distributed more densely, requiring base stations with a smaller volume. A volume of a radio frequency front-end filter module in an RFU (radio frequency unit) or an RRU (remote radio unit) of a base station is relatively large, thereby requiring a filter with a smaller volume. Considering communication quality, performance (such as insertion loss, suppression, and a power capacity) of the filter needs to remain unchanged after the volume is reduced.
Radio frequency filters have developed for decades, and a variety of filters emerge in various forms; relatively common implementation forms are a metal coaxial cavity, a transverse electric (TE) mode dielectric cavity, a transverse magnetic (TM) mode dielectric cavity, a transverse electromagnetic (TEM) mode dielectric cavity, a waveguide, a microstrip, a thin-film bulk acoustic resonator (FBAR), a bulk acoustic wave (BAW), a surface acoustic wave (SAW), and the like. Radio frequency represents an electromagnetic frequency that may be radiated to space and ranges from 300 KHz to 30 GHz.
Among the filters in various forms are filters with a relatively large volume (such as the TE mode dielectric cavity and the waveguide), filters with a relatively moderate volume (such as the metal coaxial cavity and the TM mode dielectric cavity), filters with a relatively small volume (the TEM mode dielectric cavity and the microstrip), and filters with a very small volume (FBAR, BAW, SAW, and the like). However, as analyzed from the perspective of a basic electromagnetic theory, a filter with a smaller volume causes a larger surface current, a larger loss, and a lower power bearing capability, namely, a smaller power capacity. In conclusion, a filter with a smaller volume has worse performance (loss, suppression, a power capacity, and the like).
According to a requirement of a wireless base station on performance (including insertion loss, suppression, and power) of the filter, the metal coaxial cavity, the TE mode dielectric cavity, and the TM mode dielectric cavity are commonly used currently, and the metal coaxial cavity is most commonly used. Other miniaturized filters such as a TEM mode dielectric filter and the FBAR cannot be applied to the radio frequency front-end of a large-power base station because a performance indicator of the miniaturized filters cannot meet a requirement.
At present, there is a miniaturized filter, which uses a resonator formed by a metalized (for example, silver plated) solid dielectric waveguide surface (dielectric resonator for short). Generally, the radio frequency filter (including a microwave filter) has a relatively strict indicator specification requirement (such as echo, insertion loss, and suppression). A resonance frequency of each resonator of a filter and coupling between resonators need to be accurate. However, due to causes such as a manufacturing size error in product design, a design error, and an error of a dielectric constant of a dielectric, the resonance frequency of the dielectric resonator is inaccurate and needs to be tuned.
A current tuning solution is generally to demetallize at least one of an upper surface or a bottom surface of the dielectric resonator by means of polishing. FIG. 1a and FIG. 1b are schematic diagrams of demetallizing the bottom surface of the dielectric resonator by means of polishing. FIG. 1a is a longitudinal section view and FIG. 1b is a bottom view, where 10 (FIG. 1a) represents a solid dielectric resonator body, 101 represents a metalized layer of a surface of the solid dielectric resonator body, and 102 represents a demetallized notch after the surface of the solid dielectric resonator body is polished. In this tuning solution, the inventor finds in the process of invention that in an assembly process of the resonator, the demetallized notch may be covered by a metalized surface of some components, and consequently the resonance frequency of the resonator changes and deviates from a tuned resonance frequency, thereby affecting working performance of the resonator.